Abstract: previous termDeltamethrin,next term a synthetic pyrethroid pesticide, potential toxic pollutant and significant direct risk to the aquatic ecosystems, was investigated in the present study for its toxic impact on adult monosex Nile tilapia on the basis of acute static bioassay test, and comparison of clinical, biochemical and haemato logical examinations with previous termdeltamethrinnext term-free control group. The 96 hLC50 value of previous termdeltamethrinnext term for monosex tilapia was 14.6 μg/L. The abnormal behavioral responses and toxic symptoms were described. previous termFishnext term exposed to the higher previous termdeltamethrinnext term concentration (15 μg/L) for 96 h showed significantly higher lymphocytes, total leucocytic and erythrocytic counts, haemoglobin percentage and packed cell volume content and significantly lower neutrophils compared to the control group. previous termDeltamethrinnext term caused adverse effects in the form of hypoproteinemia, hypoalbuminemia, hypercholesterolemia, hypoglycemia and significant increase of serum aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase activities. The results provide evidence that previous termdeltamethrinnext term pollution may have adverse impacts and was highly toxic to monosex tilapia.